1. Field of the Invention
The present invention relates in general to semiconductor devices, and more particularly to a means and method for providing a vertical power device having an insulating isolation wall for electrically isolating one device from another, and having a low resistance path to the conduction region of the device.
2. Description of the Prior Art
A vertical power semiconductor is an electronic device used to control the flow of electrical power. When a vertical power semiconductor device is fabricated in a semiconductor substrate, the device usually requires electrical isolation from other regions of the substrate. The purpose of such isolation is to separate the vertical power device from other components on the same chip or wafer, or to create an electrically inactive protection area at the border of the device, where the several devices or chips on a wafer may be cut apart from one another.
One solution to the problem of isolating vertical power devices is described in U.S. Pat. No. 6,579,782 (the '782 patent) issued to Roy on Jun. 17, 2003, which is hereby incorporated herein by reference. The '782 patent describes a method for manufacturing a four-layer vertical power component, such as the four-layer thyristor shown in the simplified cross-sectional view of FIG. 1 herein. A thyristor is an electronic device used to control the flow of electrical power. It is similar to a diode, but it has an extra terminal for receiving a signal that places the device in a conductive state. The thyristor is formed in a silicon semiconductor substrate 20 that is lightly doped with an N-type dopant. It has a four-layer structure consisting of alternating P-type and N-type materials (e.g., PNPN). The thyristor is surrounded at its external periphery with an isolating wall, which is formed with a material of a conductivity type opposite to that of the substrate.
Manufacturing the prior art thyristor begins with the step of forming, on the lower surface 22 of the substrate, a succession of holes 24 perpendicular to the surface. Some of the holes—the holes around a perimeter of a conduction region—form a lower portion of an isolation wall. The holes within the perimeter form the conduction region. Next, a dopant having a second conductivity type, which is opposite to that of the substrate, is diffused from both types of holes. Further steps include: boring similar holes 26 on the upper surface 28 of the substrate to define an upper portion of the isolating wall; and diffusing, from holes 26 a dopant of the second conductivity type with a high doping level, wherein the holes 24 and 26 of to the isolating wall are sufficiently close for the diffused areas to join laterally and vertically.
In the prior art, the same type of dopant is diffused from both isolation wall holes and conduction region holes. This process works fine for making four-layer devices, but an improved process is needed for making other devices, such as bipolar transistors and power MOSFETs. Such three-layer devices require isolation walls having a second conductivity type, and a conduction region having a first conductivity type.